Unlock All Stages:
Begin a new game with the name "!EVAH!" (no quotes, but remember the ! points!) 

Invincibility:
Start a new game and enter B4K42 as a case-sensitive name. The "God mode" selection will now be unlocked at the options sscreen. 

All bonuses:
Start a new game and enter 105770Y2 as a case-sensitive name. Note: The code contains the number "zero" and not a capital letter "o". All character biographies, vehicle specifications, FMV sequences, and concept artwork will be unlocked. 

Multiple codes:
Start a new game and enter more than one code together to activate each of them simultaneously. For example, enter B4K42105770Y2 as a case-sensitive name. 

Cheat: Hovering:
hovering(A by default) is VERY useful. It doesnt take any energy/ammo, doesnt overheat, it makes an exelent anti-infantry weapon. Probably the most incredable thing is that it enables the walker to 'hover' over water until you let go of (A). There are a few limitations that i have found, firstly, you cant drop from far up, and you cannot go far out(several feet into the t.v. feet)